A lot of the cine and projection lenses have an internal ring that restricts the inside diameter of the barrel, and also makes them slower, to sharpen them up for their original purposes. So, if you have an internal ring smaller than the front or rear glass, you should use that diameter to calculate the f-stop.

Richard

seawolf66

11-Mar-2012, 16:06

A lot of the cine and projection lenses have an internal ring that restricts the inside diameter of the barrel, and also makes them slower, to sharpen them up for their original purposes. So, if you have an internal ring smaller than the front or rear glass, you should use that diameter to calculate the f-stop.

Richard
Richard I Thank you for your input on this matter it turns out there are two rings inside as you said but in this case the lenses are the same in this case
I took one lens out to check it out and it was great news for me: Thanks again: